Automotive vehicle window antennas, including embedded wire or silver print antennas in rear windows and windshields, have been used for many years. More recently, metal coated infrared ray reflective thin films have been used as antennas.
Several antennas have been proposed which use a wire antenna of a quarter or half wavelength that is formed in a vehicle window by a thin film or a conductive coating on or between the layers of the glass window. Such designs may include automotive antennas that have several electrically interconnected coating regions and a transparent coating in the shape of a “T”. Also, antennas that divide the conductive coating into two pieces and have the AM and FM antennas separated to reduce AM noise and improve system performance are known.
Another proposed solution is to form a slot antenna between the metal frame of a window and a conductive transparent film panel that is bonded to the window and has an outer peripheral edge spaced from the inner edge of the window frame to define the slot antenna. Examples utilize at least one edge with a conductive coating overlapping the window frame of the vehicle body to short the coating to ground at high frequencies by coupling so as to improve transmission and reception of radio frequency waves.
From an aesthetic point of view the slot antenna concept is a generally good solution because the antenna is invisible and can be used on any window. Another benefit is a heat load reduction because the slot antenna removes a small area of heated reflective coating compared to other antenna concepts. There are various technical challenges to implementing slot antennas, especially on the windshield of a vehicle. First, there is only a limited area around the window perimeter to put the antenna elements and it may be difficult to design an antenna to meet the performance requirements. Second, slot antennas are difficult to tune to a frequency band because the antenna characteristics depend on the slot dimensions. For example, the perimeter of the window defines the maximum slot length, which defines the lowest frequency application. The lowest frequency applications may not be in the frequency band of interest. Various windshield and back glass window slot antennas can cover the FM frequency band but not the TV band I (47 MHz-68 MHz). Thus, there is a need for an antenna, for example a windshield hidden antenna, with a tunable frequency band for different applications. There is also a need for a vehicle slot antenna with advanced antenna matching and frequency tuning methods that can be used to design an antenna with acceptable performance while retaining all solar benefits of the heat reflective coating and having good aesthetics.